This invention relates generally to the testing of magnets as used in magnetic disk drives, and more particularly the invention relates to the testing of bipolar magnets.
Digital data is magnetically stored on the surfaces of disks in a disk drive. The disks are rotated at high speed, and pick-up heads move across the surfaces of the disks for storing and accessing data in concentric tracks on the disk surfaces. The pick-up heads are mounted on the ends of arms extending from an actuator which rotates the arms and heads to select a track. The actuator includes a voice coil which is placed in a magnetic field, and the actuator rotates in response to energization of the coil by the passage of current through the coil windings.
The energization of the coil in moving the pick-up heads to a desired track is controlled by a programmed computer. The program is designed to tolerate some non-uniformity in the magnets. However, it is desireable to ascertain the characteristics of the magnets prior to use in a disk drive. Moreover, by measuring the magnet characteristics, manufacturers of the magnets can modify the manufacturing processes to optimize the characteristics.
Heretofore, a pair of unipole (e.g. a pole on each of two major surfaces) magnets have been used in driving the actuator coil. More recently, single bipolar magnets have replaced the pair of unipolar magnets. While the characteristics of a unipole magnet can be measured through use of a Helmholz coil, the Helmholz coil cannot be used with a bipolar magnet due to the cancelling affects of the poles. Further, bipolar magnets are unique in having a transition from one pole orientation to another on each major surface. The physical location of this transition on the surface of a magnet can vary. Additionally, a neutral or null zone surrounds the transition line due to domain disorientation on each side of the transition line which results in decreased magnetic performance of that area of the magnet.
Characteristics of magnets are established by the magnet manufacturing processes including rare earth powder blending, pressing, sintering and heat treatment, coating, and magnetizing. By monitoring the magnetic field strength, transitional line location, and neutral zone width, the manufacturing processes can be altered to attain optimal characteristics.